Conformal coating of printed circuit boards (PCBs) has resulted in a steady evolution in performance and protection for modern electronics. It has allowed for lightweight and reliable controls in industries such as automotive and aerospace among others. This has brought about the possibility of increased length of warranties as well as fewer failures and claims. Nevertheless, original equipment manufacturers (OEMs) and consumers continue to raise their standards and expectations for longevity and reliability to levels unimagined just 10 years ago. In the automotive industry, as an example, thermal cycling testing might have involved as few as 10 or 50 cycles in the recent past. That standard is now often 500 or even 1000 cycles with no apparent ending in sight.
So what do I do when conformal coating is not enough?
Have you or your engineers either heard or asked some of the following?
- “We are seeing failures and corrosion on high profile pins and parts where there does not seem to be adequate coating.”
- “We do not want to coat over BGAs and QFNs due to the risk of wicking under the device. How do we protect the device leads?
- “Is there a method or approach other than masking to avoid our conformal coating flowing through larger holes and vias?
- How do we protect taller devices such as large capacitors and batteries from solder failures caused by vibration?
There is good news! In each of these cases the answer might be something quite simple and readily available.
What are protective gels?
Protective gels for PCBs have been developed in recent years in response to many of the conditions noted earlier to solve unique problems in PCB reliability. These gels are in most cases based on the same or similar chemistry to traditional conformal coatings including acrylics, urethanes, and UV curable acrylated urethanes. The newer gels’ main feature is dramatically higher viscosity, often in the hundreds of thousands of centipoises or most commonly thixotropic in nature. They are usually dispensed either by hand application using syringe-type packaging or alternatively, applied through a needle-type valve on selective conformal coating machinery. They are applied to taller, sharper, or sensitive components either in linear non-sag strips or spot coverage.
Why would I use a protective gel?
As discussed, the primary difference in properties between conformal coatings and gels is the large change in viscosity. Gels are thick and have high viscosity and are designed to stay in place. On the other hand, low viscosity is a necessity for conformal coatings in order for them to flow through traditional pumping and spray equipment and to conform to the PCB consistently and evenly. (Usually at 1 to 5 mils thickness).
Unfortunately, the property of low viscosity, which is necessary for processing, can at times be a significant weakness of conformal coatings. These coatings are excellent at evenly coating horizontal or flat surfaces, but they struggle to adhere in adequate amounts to taller vertical components and pins. The result is little or no protection in vertical surfaces and pins. Another common issue can be excessive flow of the coating underneath sensitive parts such as BGAs and QFNs. In each of these cases the low viscosity of the coating is a hindrance to the protection desired.
Potential Gel Applications
| Coating of pins and taller leads | 
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Coating of through-holes to prevent flow through. |
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As a supplier of most forms and chemistries of electronics and industrial coatings, Chase and Resin Designs can help you with an unbiased approach to evaluating your application and process. We’ll show you how to maximize efficiency, minimize cost, and improve product reliability. Our outstanding manufacturing and technical support groups can provide your organization with reliable global supply, unmatched quality, and superior technical support.
Contact us if you have any question.
